1. Field of the Invention
The present invention relates to a magnetoresistance effect element and a magnetic head. More particularly, the present invention relates to a magnetoresistance effect element having a BMR (Ballistic Magneto Resistance) effect and a magnetic head.
2. Related Art
In the related art, the GMR (Giant Magnetoresistance) effect is a phenomenon in which a large magnetoresistance change is exhibited when an electric current is applied to a plane of a laminated structure having a ferromagnetic layer, a non-magnetic layer and a ferromagnetic layer. A GMR element has been investigated actively in pursuit of achievement of a higher magnetoresistance ratio (hereinafter referred to as MR ratio).
A related art ferromagnetic tunnel junction or a CPP (Current Perpendicular to Plane) type GMR element for perpendicularly applying an electric current to a laminated structure has been developed for possible use as a magnetic head or a magnetic random access memory (MRAM).
Additionally, in the field of magnetic recording technology, recording bit size has been reduced with improvements in recording density. As a result, it has become more difficult to obtain sufficient readout signal strength. Therefore, there is a related art need for a material exhibiting a more sensitive magnetoresistance effect. Further, there is also a related art need for development of an element exhibiting a high MR ratio.
Recently in the related art, a “magnetic nanocontact” having two nickel (Ni) wires butted against each other was disclosed to exhibit a magnetoresistance effect of 100% or higher (e.g. see N. Garcia, M. Munoz, and Y.-W. Zhao, Physical Review Letters, vol. 82, p 2923 (1999)).
More recently, in a related art magnetic nanocontact, two fine Ni wires were arranged in a T-configuration and a nanocolumn was grown in a contact portion between the Ni wires by a related art electrodeposition process (e.g. see N. Garcia et al., Appl. Phys. Lett., vol. 80, p 1785 (2002)). The resulting related art element exhibited a very high MR ratio due to spin transport of magnetic domains present in the magnetic nanocontact formed between the two ferromagnetic layers having antiparallel directions of magnetization.
A related art magnetoresistance effect element using the above-described related art magnetic nanocontact is called a Ballistic Magneto Resistance (BMR) element because electrons pass through the magnetic nanocontact (ballistically) without change of their spin direction.
For example, Japanese Patent Laid-Open No. 2003-204095 discloses a magnetoresistance effect element including a first ferromagnetic layer, an electrically insulating layer, and a second ferromagnetic layer, wherein a hole having an opening with a maximum width of not larger than 20 nm is provided in a predetermined position of the electrically insulating layer so that the first and second ferromagnetic layers can be connected to each other. In addition, International Patent Publication No. 510911/1999 discloses a magnetoresistance effect element, including two magnetic layers connected to each other through a narrow portion having a width of about 100 nm.
G. Tatara et al., Physical Review Letters, vol. 83(10), p 2030 (1999) (hereafter “Tatara”) discloses the theoretical consideration about the magnetoresistance effect of this type BMR element. Tatara discloses a proposed magnetoresistance ratio (MR ratio) is given by Expression 1. In Expression 1, F(P, t) is the probability that electrons will reflect on a magnetic wall formed in the nano-junction, F(P, t) takes a value in a range from 0 to 1, and P is the electron spin polarization of the material for forming the nano-junction, and has a value in a range of from 0 to 1. In the expression 1, t is the thickness of the magnetic wall. The disclosed Expression 1 apparently indicates that a high magnetoresistance ratio (MR ratio) can be obtained when the electron spin polarization of the material for forming the nano-junction is increased.
                    [                  Expression          ⁢                                          ⁢          1                ]                            (        1        )                                BMR        =                                            π              2                        4                    ⁢                                    P              2                                      1              -                              P                2                                              ⁢                      F            ⁡                          (                              P                ,                t                            )                                                                      
When the aforementioned BMR element is used in a magnetic head, the related art BMR element must be produced at practicable cost, and the BMR element must have a higher MR ratio in terms of characteristic.
However, most disclosed related art BMR elements have complex element structures, because the related art BMR elements are developed with an emphasis on their characteristic nature.
Further, according to the theoretical consideration disclosed in Tatara's work, it can be conceived that the method of forming the electron spin polarization of the material of the nano-junction is effective in obtaining a high magnetoresistance ratio that approaches 1. However, no related art method has been proposed for the BMR element having a simple element structure that can be produced.